1. Field of the Invention
The present invention relates to methods of producing coated members whose base material surfaces are at least partly coated with a diamond-like carbon (DLC) film.
2. Description of the Related Art
To improve the fuel economy of an automobile, it is necessary to reduce friction of sliding portions formed on various components provided in the automobile. To do so, the surface of a base material that forms the sliding portions may be coated with a PLC film having low friction and wear resistance (high hardness) properties. The DLC film is formed, for example, by a plasma chemical vapor deposition (CVD) method. That is, a processing chamber for storing base materials is evacuated, and then a feedstock gas that includes hydrogen gas, argon gas, and hydrocarbons such as methane is continuously inducted in the processing chamber while the pressure in the processing chamber is maintained at a prescribed pressure. Then, voltage is applied to the base material in order to produce plasma in the processing chamber. Accordingly, ions and radicals are generated from the feedstock gas, and chemical reactions are initiated in order to deposit a DLC film, mainly composed of carbon (C), on the surface of the base material.
Plasma CVD methods employed here include a direct plasma CVD method and a direct-current (DC) pulse plasma CVD method in which DC pulse voltage is applied. A problem with the DLC film is that adaptation in an initial use period (hereinafter referred to as “initial adaptation”) is bad. This is likely because the surface of the DLC film lacks smoothness just after it is formed, and it is difficult to gain lubricity later on either. If initial adaptation of the DLC film is bad, the DLC film would be separated by friction in its initial use period. Also, the friction would damage the surface layer of a member that contacts the DLC layer. For the above reasons, DLC coated parts must undergo an adaptation process in which rubbing the DLC layer in advance is repeated before being actually used.
There have been proposed improved coating structures in which a film having good initial adaptation is formed on the surface of the DLC film in order to eliminate and reduce the adaptation process. The adaptation film related techniques are disclosed for example in Japanese Patent Application Publication No. 2007-162099 (JP-A-2007-162099) and Japanese Patent Application Publication No. 2003-14121 (JP-A-2003-14121). In JP-A-2007-162099, a structure in which a graphite layer is formed on the DLC film surface is proposed. In SP-A-2003-14121, a structure in which another DLC film containing silicon (Si) of 50 to 70% by mass is formed on the DLC film is proposed.
The both of JP-A-2007-162099 and SP-A-2003-14121 require a process of forming a DLC film on the base material surface and a process of forming an adaptation film on the DLC film surface, which increases the number of processes needed for producing a single coating member. It increases energy consumption and reduces productivity of coated member because of the increased production time needed along with the increase of the number of processes. Also, because the structures of JP-A-2007-162099 and JP-A-2003-14121 are both formed by laminating two discontiguous layers that differ in physical properties, such as thermal expansion coefficient and hardness, a sliding portion for example would suffer from separation and cracks on any layer, and a soft adaptation film would suffer particularly from abrasion at an early stage.